Unsupported value for UPDATE_SNAPSHOTS: {unknown:?}

Error message

Unsupported value for UPDATE_SNAPSHOTS: {unknown:?}

What it means

The UPDATE_SNAPSHOTS environment variable controls snapshot behavior (Test, UpdateFailing, UpdateAll). SnapshotOptions::from_env parses known string values and panics on anything unrecognized, refusing to guess the developer's intent. Only "false"/"0"/"no"/"off", "true"/"1"/"yes"/"on", and "force" are accepted.

Source

Thrown at crates/egui_kittest/src/snapshot.rs:400

#[derive(Clone, Copy, Debug, PartialEq, Eq)]
enum Mode {
    Test,
    UpdateFailing,
    UpdateAll,
}

impl Mode {
    fn from_env() -> Self {
        let Ok(value) = std::env::var("UPDATE_SNAPSHOTS") else {
            return Self::Test;
        };

        match value.as_str() {
            "false" | "0" | "no" | "off" => Self::Test,
            "true" | "1" | "yes" | "on" => Self::UpdateFailing,
            "force" => Self::UpdateAll,
            unknown => {
                panic!("Unsupported value for UPDATE_SNAPSHOTS: {unknown:?}");
            }
        }
    }

    fn is_update(&self) -> bool {
        match self {
            Self::Test => false,
            Self::UpdateFailing | Self::UpdateAll => true,
        }
    }
}

/// Image snapshot test with custom options.
///
/// If you want to change the default options for your whole project, it's recommended to create a
/// new `my_image_snapshot` function in your project that calls this function with the desired options.
/// You could additionally use the
/// [disallowed_methods](https://rust-lang.github.io/rust-clippy/master/#disallowed_methods)

View on GitHub (pinned to 441971a776)

Solutions

Set UPDATE_SNAPSHOTS to one of: false/0/no/off, true/1/yes/on, or force
Check for stray whitespace, quotes, or casing in your env var definition (echo "$UPDATE_SNAPSHOTS" | cat -A)
In CI, fix the workflow env block or command-line prefix to use an exact accepted value
Unset the variable entirely to use the default (Test) mode

Example fix

// before
UPDATE_SNAPSHOTS=all cargo test
// after
UPDATE_SNAPSHOTS=force cargo test

Defensive patterns

Strategy: validation

Validate before calling

const VALID: [&str; 12] = ["false","0","no","off","true","1","yes","on","force"];
let v = std::env::var("UPDATE_SNAPSHOTS").unwrap_or_default();
assert!(v.is_empty() || VALID.contains(&v.as_str()), "UPDATE_SNAPSHOTS must be one of {VALID:?}, got {v:?}");

Try / catch

// from_env panics; validate the value in CI before cargo test
match std::env::var("UPDATE_SNAPSHOTS") {
    Ok(v) if !["false","0","no","off","true","1","yes","on","force"].contains(&v.as_str()) => {
        panic!("fix UPDATE_SNAPSHOTS={v:?}: allowed values are false/0/no/off, true/1/yes/on, force");
    }
    _ => {}
}

Prevention

Use the exact lowercase accepted values in CI config and shell scripts
Quote env values carefully in shell to avoid trailing whitespace
Do not copy env var values from other snapshot-testing libraries
Add a CI lint step that validates UPDATE_SNAPSHOTS before running tests

When it happens

Trigger: Setting UPDATE_SNAPSHOTS to any string outside the accepted set — e.g. UPDATE_SNAPSHOTS=yes\ (trailing whitespace), "YES" (case sensitivity), "update", "all", or "True" — before running kittest-based tests.

Common situations: Typo'd or case-mismatched env values in CI configuration; copying "UPDATE_SNAPSHOTS=all" from another snapshot library's docs; shell quoting errors introducing stray whitespace or quotes into the value.

Understand the failure class

Background: "is not a valid" / "Invalid ... value" environment variable errors: how libraries validate env vars and what to do when they reject yours — this error's family across 48 libraries.

Related errors

AI-assisted analysis of emilk/egui@441971a776 (2026-09-12). Data as JSON: /api/errors/b4ef5cb6d8eb3b49. Report an issue: GitHub.